1. Field of the Invention
The subject invention relates to an improved process for the controlled oxidation of hydrocarbons. More particularly, the subject invention relates to the partitioning of oxygen from an aqueous solution into the hydrocarbon to be oxidized, and recovery of excess oxygen from oxidized hydrocarbon.
2. Description of the Related Art
The controlled oxidation of hydrocarbons, in particular aliphatic and cycloaliphatic hydrocarbons, represents an important industrial means for the preparation of many oxygenated organic compounds including alcohols, aldehydes, ketones, and carboxylic acids, to name but a few. For example, cyclohexane is useful as a feedstock to produce a wide range of products through partial oxidation including cyclohexanol, cyclohexanone, adipic acid, and e-hydroxycaproic acid. These products may then serve as intermediates in the preparation of such widely varied products as hexanediol and caprolactam.
Other hydrocarbons which are frequently oxidized to form oxygenated products include butane, hexane, cyclohexene, benzene, and naphthalene. It is fair to say that oxygenation processes involving these and other hydrocarbons produce a fair proportion of the commodity organic chemicals in the marketplace.
Unfortunately, the controlled oxidation of hydrocarbons, which generally takes place at elevated temperatures, presents severe safety problems. The direct introduction of oxygen, either in its pure state, as atmospheric oxygen, or as enriched atmospheric oxygen, into any hot, flammable liquid creates the danger of fire or explosion. This is especially true when oxidation takes place in the vapor phase or where the apparatus utilized allows oxygen containing hydrocarbon vapors to accumulate in the reactor head space. In the latter case, pure oxygen represents a greater danger than air, because the range of explosive limits is substantially wider. The fact that highly unstable peroxides and hydroperoxides often comprise a significant proportion of the oxidate enhances the dangers inherent in these processes.
In conventional oxidation, air is admitted to the bottom of the reactors as fine bubbles, and the headspace is carefully monitered for the presence of unreacted oxygen. In European application EP-A-0199 339, pure oxygen is used, and direct initial contact between oxygen and hydrocarbon is avoided by admitting oxygen in the form of a fine dispersion into a lower aqueous layer. The hydrocarbon to be oxidized is admitted into the oxygen solutioning vessel above the aqueous layer. However, this process has a disadvantage in that, although initial contact between oxygen and hydrocarbon is prevented by careful control pure gaseous oxygen is still physically present in the solutioning vessel and thus a greater danger of explosion exists.